Belt conveyors are used in a wide range of applications for transporting both individual packages and bulk material such as grain, ore, gravel, and the like. Such belt conveyors typically comprise a top conveying run on which the load is placed, and a bottom return run. The ends of the belt are connected together by a belt lace or like device to form an endless belt mounted on end rollers at each end of the belt conveyor.
Typically package belt conveyors have a flat top conveying run supported on a plate or rollers, while bulk belt conveyors have outer edges of the belt on the top conveying run raised to form a trough to retain the bulk material on the belt. The top conveying run may be supported in the troughed configuration by rollers or plates, and in many applications the conveying run passes through a tube that supports it in the troughed configuration. At the ends of the conveyor with either a troughed or flat conveying run, the belt is flat as it passes over the end rollers, and typically the return run is flat and is supported by idler rollers, plate sections or the like spaced at suitable intervals under the conveyor.
The mechanisms used to drive the belt will vary depending on the conveyor length, load, configuration, and like conditions. In a simple configuration, a motor can be used to drive one of the end rollers of the conveyor. Friction between the belt and drive roller causes the belt to move with the surface of the roller. Where increased force is required to drive the belt because of long length, high load, or the like, the return run typically passes through an S-drive mounted under the conveyor. In the S-drive, idler rollers cause the belt to wrap farther around the drive roller, increasing the frictional force between the belt and drive roller and allowing more power to be transferred from the drive roller to the belt.
A problem with belt conveyors is that the belt must be tracked properly, meaning it must be maintained in alignment with the end rollers, idler rollers, and S-drive rollers. Such belts tend to move laterally along the surface of the rollers toward one roller edge or the other where the belt edge will rub against support brackets and the like and be damaged. Typically adjustments are provided to track the belt by moving the ends of the rollers forward or rearward. In an initial adjustment the end rollers and S-drive rollers are squared with the proper belt path, such that the belt should track properly. During use however, conveyor belts have a tendency to wander laterally and frequent tracking adjustment is required in prior art belt conveyors. Such adjustment is conventionally accomplished by moving one end of an end roller, or one end of a drive roller, to tighten or loosen the belt.
This tracking problem has been well recognized in the prior art. U.S. Pat. No. 779,666 to Robins Jr. for example provides guide rollers oriented perpendicular to the edges of the belt on the conveying run and return run of a belt conveyor. When the belt moves laterally the belt edges contact the guide rollers and inhibit the lateral movement. The belt edges however tend to curl upward and climb the guide rollers, allowing the belt to move out of alignment and causing damage to the belt.
U.S. Pat. No. 4,917,232 to Densmore discloses flanged guide rollers oriented to rotate about axes perpendicular to the belt edges and spaced along the conveying and return runs of a conveyor belt following a curved path. Lateral movement of the belt is inhibited when the edges of the belt contact and rotate the rollers. The flanges extend over the edges of the belt to prevent the edges of the belt from curling up and climbing the guide rollers.
U.S. Pat. No. 6,390,289 to Hoggan discloses, contrary to the guide rollers of Densmore, flanged guide rollers spaced along the conveying and return runs of a conveyor belt but oriented to rotate about axes parallel to the belt edges. The flanged rollers inhibit lateral movement of the belt when the edges of the belt contact the flanges extending downward. In Hoggan the rollers extend over the edges of the belt to prevent the edges of the belt from curling up when they contact the flanges.
Prior art flanged guiding rollers used to maintain tracking provide resistance to lateral belt movement only at discrete points along the belt edge. Providing such guide rollers is costly both in terms of initial cost and continued maintenance. Further, the top conveying run of a bulk conveyor is often enclosed in a tube, such that guide rollers cannot be conveniently provided on that portion of the belt.
Rather than forcing the belt to track properly using guide rollers, U.S. Pat. No. 2,655,251 to Bankauf discloses conveyor rollers that skew in response to lateral belt movement in order to maintain the belt in the center of the rollers.